This invention relates generally to communications, more particularly, to the multiplexing of signals for transmission.
In systems where multiple signals are desired to be transmitted simultaneously within a given frequency band, some form of signal combination is required. When the signaling formats of the multiple signals are similar, simple frequency division multiplexing is a convenient solution. However, any proposed solution becomes more complicated when different signal formats are used. Some signaling formats that require different handling are: spread spectrum, single-tone QAM, multi-tone QAM, and analog modulation schemes.
In particular, consider the proposed In Band On Channel (IBOC) scheme for Digital Audio Broadcasting (DAB). In switching to the DAB system, it is envisioned that there will be a period of time (a transition period) during which both analog FM signals and digital FM signals co-exist. This spectral allocation is illustrated in FIG. 1. In FIG. 1, the triangular spectrum is produced by standard FM modulation as is currently used in the commercial FM band (88-108 Mhz), wherein stands for carrier frequency. The two rectangular sidebands are envisioned to be digitally modulated multi-tone versions of Quadrature Phase Shift Keying (QPSK). Thus, this DAB transition period requires the frequency division multiplexing of analog and digitally modulated signals. (The analog and digital bands simulcast the program material so that this IBOC approach will be compatible with current analog FM receivers.)
Currently, there are two schools of thought on how to mix multiple signals in a single transmission. These methods can be described as pre-amplification and post amplification mixing.
The pre-amplification mixing method is illustrated by FIG. 2. As shown in FIG. 2, combiner 20 provides a combined signal 3, which is the combination of the analog (1) and digitally (2) modulated signals. Combined signal 3 is applied to RF amplifier 25, which drives antenna 30. Although is it relatively simple to combine these signals (via combiner 20), there remains the problem of designing RF amplifier 25 to simultaneously process the two signals. In general, the amplifiers currently used in FM transmissions are of the class C variety. More specifically, these amplifiers are not linear. While this is not a problem for the analog portion of the combined FM signal, this means that current FM amplifiers cannot effectively pass a QAM (or digitally modulated) type signal with any true fidelity. Further, current FM amplifiers have an effective bandwidth of 200 kHz (fcxc2x1100 kHz). As such, to use the architecture of FIG. 2 in a DAB system requires existing FM stations buy a new type of high power linear RF amplifier (with a bandwidth of 400 kHz (fcxc2x1200 kHz)) so that both digital and analog modulated signals can be effectively combined. In addition, any such new RF amplifier design would become obsolete once a transition to an all digital version of IBOC is desired.
The second method, post amplification, requires two amplifiers (40 and 45) and a combining network (50) rated for high power as illustrated in FIG. 3. Unfortunately, such a combining network is typically expensive and inefficientxe2x80x94significant power is lost between RF amplifiers 40 and 45 and antenna 30. Thus, there is no way for the radio station to continue to broadcast the analog FM signal with the power it uses today. Radio Stations will either have to replace the amplifier with one of higher power or suffer the loss of analog FM coverage.
As a result, during this transition period both of the above-described solutions are undesirable because of the expense or loss of coverage area incurred.
We have discovered another alternative for multiplexing disparate signals that is more cost effective than the above-mentioned approaches. In particular, and in accordance with the principles of the invention, a multiplexed signal is formed by separately transmitting each signal in such a way that the multiplexing of the signals occurs in the channel. This eliminates the need for a combiner and cost is reduced.
In an embodiment of the invention, an FM transmitter comprises an RF nonlinear amplifier coupled to a first antenna and an RF linear amplifier coupled to a second antenna. The RF non-linear amplifier and antenna transmit an analog FM signal. The RF linear amplifier and antenna transmit a digital FM signal. The FM transmitter controls the signal transmissions such that the transmitted analog FM signal and the transmitted digital FM signal are combined, or multiplexed, together in the channel itself (here, the atmosphere) to form the transition DAB signal.
As a result there is virtually no attenuation of either signal between the RF amplifier and the antenna. Further, this approach provides a low cost alternative to the earlier described approaches. The linear amplifier is not required to be very high power since it must fit within the FCC mask (which is some 20 dB below the analog signal) in the sideband. Also, there is no change to their current analog FM signal processing. It should also be noted that when the time comes to transition to an all digital approach, the FM transmitting station can simply substitute another high power linear amplifier for the class C amplifier used for the analog FM.